Switching Class D audio amplifiers have found increasing use in the industry in recent years, due to the improvements in output stage switching devices and equally in modulation and feedback control methods. The classical switching power amplifier system consists of a pulse modulator, for converting an analog or digital source into a pulse-modulated signal, which is amplified by a switching power stage. A passive demodulation filter reproduces the power modulated power signal.
Most switching class D amplifiers are based on variants of Pulse Width Modulation (PWM). Some of the challenges in switching amplifier design relates to the shortcomings of PWM, one particular is problems associated with carrier generation since it limits system bandwidth and complicates design. Additionally, a stable and robust control system design is difficult to implement.
As a remedy to this, self-oscillating amplifiers were introduced, these are commonly classified into two categories: hysteresis-controlled oscillation and phase-shift controlled oscillation. An example of the latter in the form of a non-hysteresis controlled oscillation modulator (COM) is disclosed in WO 98/19391, by the present applicant, ICEpower A/S, incorporated herein by reference. Other oscillating modulator methods and systems have been disclosed in prior art as WO 2004/47286 by the present applicant, ICEpower A/S. These methods are characterized by self-oscillation being determined by feedback after the output filter, i.e. having the output filter as an integral, determining part on self-oscillation conditions. Such architectures will in the following be referenced to as global loop controlled oscillation modulators (GCOM). The global loop controlled oscillation modulator based switching amplifier systems disclosed in prior art have a particular advantage in terms of wide closed loop gain bandwidth enclosing the output filter, such that filter distortion and output impedance is minimized. Moreover, the (resulting/achieved) open loop gain of GCOM is defined as the difference between the open loop gain of the audio operational frequency which is generated and the open loop gain at the actual switching frequency. Therefore, it is desirable to have as much open loop gain as possible in the operational range and as much open loop attenuation as possible outside of the operational range.
Thus, there is still a need for improvement in the art, particularly there is a need for switching power conversion systems with improved closed loop bandwidth and reduced phase shift across the audio band.